Plasma from two patients with antibodies to von Willebrand factor are being studied to identify the epitopes on von Willebrand factor that are recognized by the antibodies. The general approach has been to enzymatically cleave vWf and then immunoprecipitate the resulting fragments using patient immunoglobulin covalently coupled to protein A - Sepharose beads. Following digestion of vWf by thrombin, numerous fragments ranging in size from about 176 kD to 40 kD are generated. Preliminary experiments using a burro polyclonal antibody have not been sucessful in immunoprecipitating any vWf fragments. It is unclear as to why, but possible explanations include destruction of the epitopes by digestion and insufficient protein to be detected by immunoblotting. Plasma from two patients with antibodies to vWf will be used in these studies when it is clear that the vWf fragments can be immunoprecipitated by this method.